1. Field of the Invention
The present invention relates to a transmission apparatus installed on a vehicle such as for example a motor vehicle.
2. Description of the Related Art
In general, a motor vehicle is provided with a transmission for converting the power of an engine into a torque and a rotation speed corresponding to the operating state of the engine. As such a transmission, there have been proposed a manual transmission that changes the transmission gear ratio by shifting gear trains by a manual operation, an automatic transmission that changes the transmission gear ratio by automatically shifting gear trains in accordance with the operating condition of an engine by using a driving unit, and so on.
In addition, in recent years, regulations or restrictions on fuel mileage or economy is becoming severe, and hence improvements in fuel mileage is increasingly required.
Accordingly, there has been proposed an automatic transmission that is able to perform automatic transmission by attaching actuators to a manual transmission and a clutch, respectively. Such an automatic transmission is provided with gear trains including plural stages and a change-over or shift mechanism that shifts the gear trains, so that an actuator makes the change-over mechanism perform a shift operation and a selection operation thereby to shift the gear trains and to change the transmission gear ratio.
Moreover, for a forward drive first-speed gear train or a driving reverse (back) gear train among the gear trains with plural stages in the automatic transmission, there is employed a gear train that does not use any synchronization mechanism (hereinafter referred to as “the nonsynchronized gear train”) for synchronizing the rotation speeds of individual gear wheels.
In such an automatic transmission, when a shift is made to the nonsynchronized gear train (the forward drive first-speed gear train or the driving reverse gear train), the vehicle is standing still, and hence the output shaft of the automatic transmission is stopped. In addition, the input shaft of the automatic transmission becomes equal to or slightly lower than the idling rotation speed of the engine (i.e., about 800 rpm) because immediately after the clutch has been turned off.
Here, note that there is a feature that a shift to the nonsynchronized gear train is easy due to the shapes of the gear wheels when the rotation speed of the input shaft is in a range of from 250 to 350 rpm.
Therefore, in case where the gear is shifted to the nonsynchronized gear train, a gear change or shift might not be made due, for example, to the occurrence of tooth contact of the gear wheels in which the crests of the gear wheels are placed in contact with each other when the rotation speed of the input shaft is too low, whereas when the rotation speed of the input shaft is too high, the gear wheels might become unable to come into engagement with each other in a smooth manner, generating gear sounding in which sound or noise is generated.
In view of such a situation, the conventional automatic transmission apparatus for a vehicle uses a gear transmission and a mechanical clutch, and drives them under electronic control so as to perform a gear change operation to a requested shift stage or speed in an automatic manner. Also, the apparatus includes, as shift stages of the gear transmission, a synchronized stage (a gear train using a synchronization mechanism) and a nonsynchronized stage (a nonsynchronized gear train), and has a nonsynchronized shift control part and a retry shift control part.
Under such a condition, when the clutch is turned off or disconnected upon generation of a request for shift to the nonsynchronized gear stage, the nonsynchronized shift control part starts shifting the transmission to the synchronized stage. When a rotation speed difference between a countershaft (input shaft) and a main shaft (output shaft) decreases up to an upper limit value at which gears can be placed into synchronous meshing engagement with each other, a shift to the nonsynchronized stage is carried out after returning the transmission to a neutral position or stage. When the rotation speed difference between the main shaft and the countershaft is reduced below a lower limit value for avoiding the stopping of the countershaft rotation during such control, the retry shift control part returns the transmission to the neutral stage, and at the same time, temporarily connects or turns on the clutch thereby to increase the rotation of the countershaft, and then disconnects or turns off the clutch whereby a shift to the nonsynchronized stage is carried out again (see, for example, a first patent document: Japanese patent application laid-open No. H11-105585).
Hereinafter, reference will be made to the operation of the conventional apparatus as described in the above-mentioned first patent document while referring to an explanatory view of FIG. 10.
In FIG. 10, first of all, when a request for shift to the nonsynchronized stage (the driving reverse gear train) is generated, the clutch is released at time t11, and the nonsynchronized shift control part then starts shifting the transmission to the synchronized stage at time t12.
Subsequently, at time t13 at which the rotation speed difference between the countershaft and the main shaft decreases to the upper limit value at which the gears can be put into synchronous meshing engagement with each other, the nonsynchronized shift control part returns the transmission to the neutral stage.
Then, at time t14 at which the transmission has been returned to the neutral stage, the nonsynchronized shift control part performs a shift of the transmission to the driving reverse gear train, and at time t15, the shift operation is completed.
In the conventional automatic transmission apparatus for a vehicle, however, there is the following problem. That is, it is impossible to perform the reduction of the rotation speed of the countershaft due to a shift to the synchronized stage and a shift operation to the nonsynchronized stage in parallel with respect to each other in view of the construction thereof, so a long time is required for a shift operation from the start to the end or completion of the gear shift.
In addition, there is also another problem that it is necessary to return the transmission to the neutral stage when a shift is made from the synchronized stage to the nonsynchronized stage, so the rotation speed of the countershaft might become irregular, thus generating tooth contact or gear sounding